Cosmogenic 3He Age Estimates of Plio-Pleistocene Alluvial-Fan Surfaces in the Lower Colorado River Corridor, Arizona, USA

Quaternary Research 79 (2013) 86–99

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Cosmogenic 3He age estimates of Plio-Pleistocene alluvial-fan surfaces in the Lower Colorado River Corridor, Arizona, USA

Cassandra R. Fenton a,⁎, Jon D. Pelletier b a Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum, Telegrafenberg, D-14473, Germany b Department of Geosciences, University of Arizona, 1040 E Fourth St., Tucson, AZ, 85721, USA article info abstract

Article history: Plio-Pleistocene deposits of the Lower Colorado River (LCR) and tributary alluvial fans emanating from the Received 9 May 2012 Black Mountains near Golden Shores, Arizona record six cycles of Late Cenozoic aggradation and incision of Available online 22 November 2012 3 3 the LCR and its adjacent alluvial fans. Cosmogenic He ( Hec) ages of basalt boulders on fan terraces yield age ranges of: 3.3–2.2 Ma, 2.2–1.1 Ma, 1.1 Ma to 110 ka, b350 ka, b150 ka, and b63 ka. T1 and Q1 fans are Keywords: especially significant, because they overlie Bullhead Alluvium, i.e. the first alluvial deposit of the LCR since Colorado River its inception ca. 4.2 Ma. 3He data suggest that the LCR began downcutting into the Bullhead Alluvium as basalt c cosmogenic 3He early as 3.3 Ma and as late as 2.2 Ma. Younger Q2a to Q4 fans very broadly correlate in number and age 3 surface exposure ages with alluvial terraces elsewhere in the southwestern USA. Large uncertainties in Hec ages preclude a tempo- climate change ral link between the genesis of the Black Mountain fans and specific climate transitions. Fan-terrace morphol- desert pavement ogy and the absence of significant Plio-Quaternary faulting in the area, however, indicate regional, episodic desert alluvial fans increases in sediment supply, and that climate change has possibly played a role in Late Cenozoic piedmont southwest USA geomorphology and valley-floor aggradation in the LCR valley. © 2012 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction vegetation cover during humid-to-arid transitions increase the sediment supply to piedmonts and valley-floor channels adjusted to a Fluvial systems in the southwestern USA often record multiple epi- lower sediment supply, thereby triggering aggradation. After the transi- sodes of Plio-Quaternary aggradation and incision (Haynes, 1968; Bull, tion, i.e. when the climate has stabilized in a drier state, sediment supply 1991). These events are recorded as fluvial deposits (emplaced during declines from its peak, causing valley incision and abandonment of a fill aggradation) and their bounding geomorphic surfaces (abandoned dur- terrace along piedmonts and valley floors. Adequate age control exists ing incision) preserved along valley floors and on piedmonts. Studies of for only a few locales in the southwestern USA, but in places where cycles of alluvial aggradation and incision in the southwestern USA go strong age constraints do exist, Late Quaternary alluvial terraces have back to the earliest days of geomorphology (e.g. Bryan, 1922), but ages of ca. 320 ka, 120 ka, 70–40 ka, and the Pleistocene–Holocene tran- Haynes (1968) was the first to suggest that cycles of aggradation and in- sition (e.g. see Anders et al., 2005, for a full suite of ages from eastern cision are regionally correlative and provide geochronologic evidence Grand Canyon). These ages correlate with glacial-to-interglacial for such a correlation in the Late Quaternary portion of the record. On (i.e. humid-to-arid) transitions recognized in regional climate-change piedmonts, cycles of alluvial aggradation and incision have been linked proxies (e.g. Anders et al., 2005). In the Lower Colorado River (LCR), ad- to climatic changes (Melton, 1965; Royse and Barsch, 1971; Christensen ditional mechanisms could potentially trigger cycles of alluvial aggrada- and Purcell, 1985; Bull, 1996; Reheis et al., 1996), tectonic uplift of the tion and incision, including eustatic sea-level changes (Merritts et al., adjacent mountain range (Hooke, 1972; Rockwell et al., 1985), and/or 1994) and catastrophic floods originating in western Grand Canyon the internal dynamics of the fluvial system, including upstream drain- (Fenton et al., 2004, 2006). age reorganization (Ritter, 1972) and coupled oscillations of the Several methods may be used for distinguishing among these poten- channel's bed and banks (Schumm and Parker, 1973). Knox (1983), tial triggering mechanisms. First, absolute stratigraphic and deposit working in Holocene alluvial valleys in Wisconsin, developed the ages are critically important for establishing correlations between chro- biogeomorphic response model that became the basis for Bull's nologies of aggradation–incision cycles and of potential forcing factors, (1991) suggestion that Plio-Quaternary terraces in the southwestern such as time-series data for paleoclimatic proxies. Second, stratigraphic USA are climatically generated. In Knox's model, changes in hillslope and geomorphic mapping can be used to establish a regional correlation of deposits. Third, the analysis of terrace geometries, including their dips and cross-cutting relationships with other units, enable a sequence ⁎ Corresponding author. E-mail addresses: [email protected] (C.R. Fenton), of events to be reconstructed and may provide evidence that one mech- [email protected] (J.D. Pelletier). anism may be favored over another. For example, parallel fan terraces

0033-5894/$ – see front matter © 2012 University of Washington. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.yqres.2012.10.006 C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99 87 that grade to distinct terraces of the valley-floor channel suggest a within 40 m of its modern level (Fig. 1B). As such, the Late Cenozoic his- significant base-level control by the valley-floor channel on piedmont tory of the LCR is characterized by episodic backfilling and entrench- aggradation and incision (Fig. 1; e.g. DeLong et al., 2008). Figure 1A, ment. Slope gradients of well-preserved terrace treads emanating for example, contrasts the fan terrace geometries within a coupled from the Black Mountains indicate that the alluvial fans of the Black valley-floor-alluvial-fan system that could be expected from: (1) epi- Mountains were steepening at the same time the LCR was aggrading, sodic increases in sediment supply from local drainage basins only, trig- i.e. slope gradients are significantly steeper on the older deposit (T1, gering steepening and aggradation of the fan without significant which has a gradient of 0.024) relative to the younger units, which changes to the valley-floor channel that controls the base level of the have gradients of 0.021). As such, terrace geometries suggest regional fan (shown in left diagram); (2) episodic increases in sediment supply aggradation rather than increases in sediment supply affecting only from the Upper Colorado River and Grand Canyon only, triggering in- the Black Mountains or only the LCR. creases in the bed elevation of the valley floor channel, causing the fan to aggrade in concert with the valley floor but without steepening Geological setting of the fan terraces (middle diagram); and (3) episodic increases in sediment throughout the region, triggering a combination of the responses An alluvial-fan terrace is created when a channel erodes into its in the previous two scenarios LCR (right diagram). Tectonic uplift of the deposits, creating an entrenched channel and an abandoned terrace. Black Mountains is not considered in Figure 1A because geologic map- Successive episodes of aggradation and entrenchment are recorded in ping reveals only very minor (i.e. b10 m offset) active folding/faulting many alluvial fans of the Southwest, preserving a series of terraces in the area during Plio-Quaternary time (Howard et al., 2000). that rise like a flight of stairs from the modern channel. Our study site Here we provide geomorphic descriptions, surficial geologic maps is located in Mohave Valley, between the Lower Colorado River on the and topographic analysis of alluvial fans in the Lower Colorado River west and the Black Mountains on the east, near Golden Shores, AZ corridor, adjacent to Black Mountains near the California–Arizona (Figs. 2 and 3). The Lower Colorado River is defined as the reach of (CA-AZ) border at Golden Shores, Arizona (Fig. 2). To that we add cos- the Colorado River that extends from the western edge of the Colorado 3 3 mogenic He ( Hec) surface-exposure age estimates for eight separate Plateau (Grand Canyon) to the Gulf of California. The Black Mountains fan surfaces. The Lower Colorado River and its tributary channels from are a north–south trending mountain range on the CA-AZ border, be- the Black Mountains experienced at least six synchronous aggradational tween the town of Needles and Lake Mead, created during Basin and events in the past 5.3 Ma (House et al., 2008; Lundstrom et al., 2008; Range extension between 25 and 10 Ma (Spencer and Reynolds, Malmon et al., 2011; Matmon et al., 2012). Deposition of the Bullhead 1989). The southern end of this metamorphic core complex consists Alluvium (House et al., 2005, 2008) is one of the most significant aggra- of early to middle Miocene volcanic rocks capped with mesa-forming dational periods, during which the LCR aggraded at least 200 m above basalt dated at 15.8 Ma (K–Ar; Gray et al., 1990), though other basalt the modern river level. Following each aggradation, the LCR incised to flows in the Black Mountains have reported K–Ar eruption ages of

A End-member models for terrace formation - tectoncially inactive terrain

constant base level and position, aggrading base level and position, aggrading base level and position, fluctuating sediment supply constant sediment supply fluctuating sediment supply Q1 Q1 Q1 Q2a axial channel alluvial fan base level Q2a

Q2a base level

base level

B Terrace geometry in Lower Colorardo River at Warm Springs SW 500 Height above river level (m) 400

T1 300 slope = 0.0024 Bullhead (200 m) Q1 Laughlin (165 m) 200 slope = 0.0022 Q2a Davis (100 m) Emerald (5 m) slope = 0.0022 100 Mojave (60 m) Q2b Q3 slope = 0.0021 Modern slope = 0.0021 0 m.a.r.l. Lower 15 20 5 10 Mountain Colorado Front River Distance from modern floodplain along Warm Springs Wash (km) (140 m.a.s.l.)

Figure 1. Schematic diagrams illustrating how fan-terrace geometry in a coupled valley-ﬂoor/alluvial-fan system can distinguish among regional and local triggering mechanisms. (A) Comparison of terrace geometries resulting from episodic increases in local sediment supply only (left), episodic increases in upstream sediment supply only (middle), and episodic increases in regional sediment supply (right). (B) Terrace geometry of the Black Mountain alluvial fans on the east side of the Lower Colorado River at Warm Springs, AZ. Older fan terraces are steeper and each grades to a speciﬁc terrace deposit of the LCR. 88 C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99

Figure 2. (A) Regional map of the Lower Colorado River (LCR) valley. The river ﬂows north to south. (B) LANDSAT band 3 and (C) shaded-relief images of the Late Cenozoic/Qua- ternary surfaces of the Black Mountains, AZ. The light-toned bands in Fig. 2B that parallel the Colorado River are exposures of the Colorado River deposits that comprise the Plio-Pleistocene terraces of the LCR named by House et al. (2005, 2008) as (from youngest to oldest) Mojave, Davis, Laughlin, and Bullhead.

15–20 Ma (Harris, 1998). Mountains flank the Lower Colorado River on the LCR and the Black Mountain tributaries began alternately aggrading its east side for ~150 km between Hoover Dam (impounding Lake and incising until the LCR finally reached its modern-day river level in Mead) and Golden Shores, AZ. Alluvial fans aggraded to the west from theHolocene(House et al., 2005, 2008). Alluvial-fan gravels on the these mountains and graded to current and pre-existing levels of the western flank of the Black Mountains overlie main-stem Colorado River Colorado River (Figs. 1 through 4). Fans contain both rhyolites and ba- gravels (Figs. 2 and 3). The Black Mountain fan surfaces sampled for cos- salts from their source area—the Black Mountains—but are dominated mogenic dating in this study represent periods of aggradation and inci- by basalt boulders. sion recognized in many fluvial systems throughout the southwestern It is generally agreed that the Colorado River has been a through- USA for which climatic change has been proposed as the likely driver flowing river along the CA-AZ border for at least the past 4.3 Ma (e.g. Ku et al., 1979; Weldon, 1986; Reheis et al., 1996; Zehfuss et al., (Lucchitta, 1979; Johnson and Miller, 1980; Howard et al., 2000; 2001; McDonald et al., 2003; Anders et al., 2005). Faulds et al., 2001; Howard and Bohannon, 2001; Spencer et al., 2001; Figures 2C and 3 illustrate with satellite images (Landsat and House et al., 2005, 2008; Howard et al., 2008; Matmon et al., 2012). SRTM) and a derived map the Quaternary surfaces of the fan complex There is considerable evidence (House et al., 2005, 2008) that indicates we studied. The surfaces are informally referred to as the Warm soon after the inception of the free-flowing LCR, there was a major Springs fan complex (after House et al., 2005). The Warm Springs aggradational event that resulted in deposition of >200 m of Bullhead fan complex here is approximately 20 km south of contemporaneous Alluvium in the LCR corridor between 3.6 and 4.2 Ma. Around 3.3 Ma, alluvial fans preserved in Mohave Valley near Bullhead City, which C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99 89

Figure 3. Surﬁcial geologic map of alluvial-fan and river-gravel terraces on the Lower Colorado River near the Black Mountains, AZ. Terraces are distinguished based on their elevation above active channels and soil-carbonate, desert-pavement, and rock-varnish development (Table 1 and Supplementary Table 1). Fan surfaces here and in Fig. 4 are labeled as Surface A through O, and possible correlations with Bull's (1991) T1 to Q4 scheme are listed in Table 3. Correlations with Bull's (1991) scheme are based on mapping and relative geomorphic, stratigraphic positions of fan-terrace surfaces to one another. Plio-Pleistocene terraces (i.e., Mojave, Davis, Laughlin, and Bullhead) of the LCR named by House et al. (2005, 2008).

are described and mapped in detail by House et al. (2005, 2008).In a flight of terraces of increasing age from Q4 to T0, following the con- this field study, the terraces of Figures 2–4 are distinguished based vention of Bull (1991). on their relief above active channels, and soil, desert pavement, and rock-varnish development. Field component of surface-exposure dating of fan terraces The light-toned bands in Figure 2B that parallel the Colorado River 3 3 are exposures of the Colorado River deposits that comprise the Plio- Cosmogenic He ( Hec) is produced by spallation reactions involv- Pleistocene terraces of the LCR named by House et al. (2005, 2008) as ing high-energy neutrons and target elements such as O, Na, Mg, Al, 3 (from youngest to oldest) Mojave, Davis, Laughlin, and Bullhead. The Si, Ca, and Fe (Gosse and Phillips, 2001, and references therein). Hec dark-toned deposits emanating from the Black Mountains are fan de- accumulates over time and, because it is stable, it is quantitatively posits of mid-to-late Pleistocene age that have well-developed desert retained in both olivine and pyroxene for up to 25 Ma (Margerison et varnish and pavement. Holocene fan sediments lack desert varnish al., 2005; Evenstar et al., 2009). If a landform has experienced low or 3 and pavement due to insufficient time for their formation, while early negligible erosion or burial, the amount of Hec in a sample from that Pleistocene and Pliocene terraces in the study area lack desert pave- landform directly equates to the amount of time that sample has been ment; boulders with well-developed desert varnish (Supplementary exposed to cosmic rays (Gosse and Phillips, 2001, and references there- 3 Table 1) sit directly on calcrete. Erosion has removed the smaller surface in). Hec has been used to determine erosion rates and exposure ages of clasts and Av horizons from these older deposits, exposing resistant a variety of Quaternary surfaces including basalt flows, flood deposits, calcrete and giving older deposits a relatively light-toned appearance desert pavements, glacial landforms, and planation surfaces (Cerling in satellite images as a result. and Craig, 1994; Laughlin et al., 1994; Wells et al., 1995; Poths et al., Each fan surface is here labeled as Surface A, B, C, etc., the order in 1996; Cerling et al., 1999; Schaefer et al., 1999; Fenton et al., 2001, which we mapped and described them during our southeast-to- 2002; Marchant et al., 2002; Fenton et al., 2004; Marchetti and northwest sample collection along the Power Line Road. This precluded Cerling, 2005; Fenton et al., 2009; Fenton and Niedermann, 2012). 3 any immediate assignment into an age-specific terrace. We used this Studies of alluvial fans using Hec are rare. Surface-exposure-age approach because correlation to the classic Bull (1991) sequence was studies of alluvial fans have typically involved cosmogenic 26Al, 10Be, not immediately straightforward and to avoid pre-judging the age and and 36Cl (Hooke and Dorn, 1992; Granger et al., 1996; Liu et al., 1996; correlation of the surfaces. Later, following the convention of Bull Robinson et al., 2000; Nichols et al., 2002; Gosse, 2003; Matmon et al., (1991), we assigned map units to the terraces based on relative strati- 2005, 2006; Machette et al., 2008). Although Fenton et al. (2001) 3 graphic position and geomorphic characteristics. Figures 5 and 6 show report Hec ages for several offset alluvial-fan surfaces in a study of 90 C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99

114.45°W 114.44°W 114.43°W

Oatman Road CT1 (Bullhead) Q2b Q4 O 34.87°N BM-101, -103 CT1 Q2b M Q1 Q2a

N L K Q2b 020903 Q2c J Q2a BM-105,106 H BM-135, I Q2c F 136 Q3 E G 020803 34.86°N D Q2b BM-138 Five Mile C Wash Q2a Q4: 110 - 140 ka Q2b(?): B Q2b 100 -160 ka Q1 N Powerline Road Q2a 100 m

Map Units CT1 B Alluvial fan-terrace surface Q2c Q1 (Bullhead) specifically documented Q2a Q3 3He sample site in this study(Tables 1 and 2) Q2b Q4 (with sample number)

Figure 4. Surﬁcial geology map of alluvial-fan terraces near Five-Mile Wash that aggraded from the Black Mountains. Alluvial-fan terraces are labeled as Surface A through O, and then assigned a map unit according to Bull (1991). Sample numbers are also mapped. See Tables 2 and 3 for details. Colorado River sediments (CT1) equivalent to the Bullhead Alluvium unit of House et al. (2005, 2008) occasionally crop out in the alluvial fans, but are too small to map separately.

J T0 A

C F D

I H

Figure 5. Photograph of a ﬂight of fan terraces in the Black Mountains at Five-Mile Wash that are capped by dominantly basalt boulders, cobbles and desert pavements. These fan surfaces were created when the channel eroded into a depositional surface, creating an entrenched channel and an abandoned surface. Photographer is looking eastward, up the wash. C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99 91

Figure 6. Schematic diagram showing relative elevations and stratigraphic positions of Surfaces A through J, and O. Estimated ranges of exposure ages from this study are also listed for each map unit where data was available. normal faulting in western Grand Canyon, this paper is the first, to the (Fig. 7; Table 1; Supplementary Table 1). Desert varnish is extremely 3 best of our knowledge, to report the use of Hec in a systematic study well-developed on clasts capping the fan Surfaces A and B (T1 and of sequential Plio-Pleistocene alluvial-fan terraces. Though we acknowl- Q1). Soil-carbonate development also increases with increasing ter- edge cosmogenic exposure dating has its limitations in alluvial-fan set- race age (Table 1). On the other hand, fan Surfaces A and B (T1 and 3 tings (Gosse, 2003), we chose Hec as the isotope of choice for this study Q1) have no Av–Bt soil horizons indicating extensive erosion; small because olivine- and pyroxene-bearing basalts from the Black Moun- (b40 cm b-axis length), cracked basalt boulders and cobbles rest di- tains are more resistant to weathering than rhyolites in the area. rectly on exposed calcrete, which cements the top several meters of these fan deposits. Mechanical weathering is more evident on older Field observations surfaces, where the frequency of cracked boulders increases with age (Supplementary Table 1). Solar insolation is potentially an impor- In order to apply cosmogenic dating to alluvial fans, a stable fan sur- tant cause of mechanical weathering in deserts (McFadden et al., face is needed. In many places in the southwestern United States, older 2005). For example, boulders from T1 and Q1 surfaces (A and B) fans have degraded to the point where hillslope and gully erosion have have experienced diurnal changes in solar insolation for more than entirely removed the terrace tread leaving only ridge-and-ravine topogra- ca. 1 Ma. These surfaces are littered with an abundance of boulders phy; however, fan deposits from the Black Mountains are underlain by that have been cracked into multiple angular pieces and fallen fine-grained river sands from the Colorado River. This coarse-over-fine apart, showing no remnants of the boulders' original forms. Boulders stratigraphy causes infiltrating water to move laterally towards gullies on Q2a surfaces have been cracked all the way through, but the halves that incise these deposits, resulting in erosion primarily by slope retreat, remain lying near each other, so ‘partner’ rocks are obvious. Boulders and broad, flat surfaces with steep-walled gullies (Fig. 5), that are unlike from Q2c surfaces have been cracked through, but the partner rocks the typical morphology of older fan terraces in the southwestern USA. still stand together in the original form of the boulder. The morphology of the Black Mountain fan terraces makes them relatively more stable, though erosion is noted (Table 1; Supplementary Table 1) and, thus, candidates for cosmogenic surface-exposure dating. Sampling strategy for exposure dating We identified and described several well-preserved alluvial-fan terraces built by the Five-Mile Wash drainage along the NW–SE-oriented We sampled the upper 4 cm of individual basalt boulders on Power Line Road located approximately 3–4 km from and nearly paral- alluvial-fan Surfaces A, B, B1, C, F, G, H, J, and O (T1 to Q4). Although sci- lel to the mountain front (Figs. 3 and 4; Table 1 and Supplementary entists recognize that the use of clast-amalgamation in depth-profiles Table 1). for collecting cosmogenic samples in alluvium typically yields more ac- The fans are composed predominantly of Tertiary basalt boulders curate surface exposure ages (Repka et al., 1997; Gosse and Phillips, and cobbles, though weathered rhyolites are also present. Boulders up 2001), amalgamation of basalt boulders or clasts in our study was not to 1 m in b-axis diameter are found, but clasts with 20–40 cm b-axis di- possible. Depth profiles were not used as the primary sampling strategy ameters dominate the fan surfaces. Clasts range from subangular to because many terraces were too cemented with soil carbonate (Table 1; rounded, but are predominantly subrounded. Clasts that were sampled Supplementary Table 1) to allow soil pits to be manually excavated. Me- for cosmogenic nuclide analysis stood at least 20 cm above their respec- chanical excavation of soil pits was not an option. Furthermore, basalt tive fan surfaces and were 10–60 cm in b-axis diameter (Table 2). Fan boulders in this field area are very resistant and in many cases, whole surfaces are mapped as Surfaces A through O along the Power Line boulders had to be collected and sawed in the laboratory to obtain the Road (Figs. 3–5). Younger fan terraces are lower in elevation and inset top 4 cm of rock suitable for exposure dating. Limited field time and against older, higher fans. The highest sampled terrace (Surface A; T1), a limited number of suitable surface samples precluded us from which is more proximal to the mountains than the other terraces, is collecting, for example, the tops of 20 boulders for each surface and 113 m above the active Five-Mile Wash where the wash crosses Power amalgamating them into one sample, in order to obtain an “average” Line Road. A separate road was used to access the Surface A (T1) terrace. surface-exposure age. Between two and four boulders from each The oldest fan surface (T0; Fig. 3) was not accessed or sampled. surface were used in cosmogenic nuclide analysis, except for Surfaces Desert-pavement and desert-varnish development generally in- F and G, from which only one sample was collected (Table 2). We crease with increasing stratigraphic age. The thickness of the Av hori- collected only from boulders that appeared to us to be stable and to zon generally decreases with decreasing age of Surfaces C through O have remained “in-place” since emplacement, noting in Table 1 and 92 C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99

Table 1 Geomorphic descriptions and exposure-age estimates of Black Mountain alluvial fans.

b Terrace Possible correlation Height above Av horizon Soil carbonate development surface with Bull (1991) active wash (m)a thickness (cm) terrace:

A T1 113 None — stripped Carbonate pendants without associated clasts as surface lag up to 15 cm in length. Clasts resting on down to Cc layer calcrete that is ~5 m thick. Pendants have pizolites and brecciation. B Q1 27 None — stripped Clasts in vertical exposure are completely coated with Cc; matrix and clasts are cemented together. down to Cc layer Pendants up to 6 cm long. Laminated Cc up to 8 cm thick. At least Stage IV with more massive Cc than Surface N. B1 Q2a 16 – Carbonate rinds on clasts up to 2 cm thick. Calcrete litter on surface up to 10 cm thick. C Q2a 7 19 No vertical exposure. Overlies Bullhead Alluvium (Colorado River sediments). D Q2b 6 6 Clast bottoms are coated in Cc 1–2 mm thick. Stages II–III? Soil is under swale; much less developed soil than that of possibly correlative Surface I. E Q2c 2 6 Clast bottoms have discontinuous Cc coatings ≤1 mm. Not all clasts have Cc coating. Weak Stage II? F Q4 0 (Active wash) 0 None; however, rare clasts have Cc coatings, indicating older clasts have been reworked into active wash. Cc coatings range from clast bottoms to entire clast G Q3 1.5 4 Weak Cc coatings on bottoms of clasts, often discontinuous. Stage I. H Q2c 3.5 5 20–30 cm of erosion. Carbonate rinds on ﬂank of rocks that are on the surface. Solid Stage III soil–carbonate development. I Q2b 7 15 Clasts completely coated in Cc. Clasts and matrix cemented together. 1.5 m depth to Cc bottom. Rare, discontinuous Cc laminations. Fewer than Surface N soil. J Q2a 10 15 Clasts on surface have Cc coatings on bottom in pendant forms that are 1–3 cm thick; pendants are litter on surface. Exhumed or washed-in Stage V Cc fragments on surface. No vertical exposure. Deﬁnitely stratigraphically younger than Surface B1. K Q2b 3 10 Cc coatings on bottom of some clasts in form of pendants 3–4 mm thick. Clasts dug up by road—no clasts completely coated in Cc. No vertical exposure. L Q2c 1 3 No Cc litter or pendants on surface; Some clasts in road have thin Cc coating on bottom (b1 mm). No vertical exposure. M Q2b 4.5 15 Very common 3–4 mm Cc pendants on bottoms of surface clasts. Clasts in vertical exposure have Cc coatings on bottoms only, but the matrix and clasts are cemented together. Cannot see bottom contact of Cc horizon. Possible weak Cc laminations. Stage III at least. N Q2a 12 5 Clasts are completely coated in Cc. Matrix and clasts cemented together. Depth to Cc is 1.55 m; rare laminations; weak Stage IV. O Q2b 11 3 Clasts on surface have weak Cc dusting on bottoms. No vertical exposure.

Note: – indicates not applicable or data not available. a Heights measured with digital Brunton altimeter and are relative to the elevation of the active wash (308 m; Surface F) where it crosses the Power Line Road. b Cc = soil carbonate; stages are described after Machette (1985).

Supplementary Table 1 desert-varnish development and other geomor- from cosmic-ray exposure, and access to the cliff faces was limited phic characteristics. due to safety concerns. No fully shielded samples were collected from Miocene basalts Previous exposure (cosmogenic nuclide inheritance) representing boulder sources in the adjacent Black Mountains, because there was no certain way to determine exactly which intact Previous exposure of a clast results in retention of remnant, or lava flow contributed specific basalt clasts in the individual terraces. inherited, cosmogenic nuclides from an earlier episode of exposure Active channel sample from Surface F (Q4). We also analyzed one (Gosse and Phillips, 2001). For example, a rock clast preserved on an clast collected from the surface of the active Five Mile Wash near the 3 alluvial-fan surface may have some residual Hec inherited from a Black Mountains (sample AZ01-BM-138; Surface F; Figs. 4 and 5; time that that clast may have been exposed on a cliff surface, a hillslope, Table 1), with the assumption that all cosmogenic 3He measured in or a pre-existing alluvial fan before its deposition on the fan of interest. this sample represents the boulder's previous exposure history. Alluvium is supplied mainly from hillslope colluvium and erosion of bedrock outcrops (Clapp et al., 2000, 2002). Incised channels derive Geochemical methods much of their sediment load from erosion of basin alluvium stored in terraces of long-inactive alluvial fans and from sediment formerly Cosmogenic 3He laboratory analyses exposed on hillslopes or bedrock outcrops (Clapp et al., 2002). Inherited cosmogenic nuclides will cause the sample to have an ap- Pure olivine or pyroxene separates were obtained from the parent exposure age older than the actual depositional or abandonment 180–425 μm fractions of crushed samples through magnetic and age of that part of the alluvial fan. Sediment storage in various heavy-liquid separation techniques. Mineral separates were treated alluvial-fan systems of the Southwest typically occurs on the order of with dilute (b5%) HCl and HNO3 to remove carbonate and iron-oxide 100 ka (Clapp et al., 2000, 2002; Nichols et al., 2002; Gosse, 2003). To coatings, and were inspected under a microscope to ensure a pure min- test the extent to which previous exposure influenced Black Mountain eral separate. Olivine and pyroxene were analyzed for 3He/4He content alluvial fan samples, we collected samples L00-3-2m, L00-3-2m1, and on a MAP 215-50 noble gas mass spectrometer at the University of Utah. L00-2-2m 2 m below the surface elevations of Surfaces A, B, and J Two mineral separates were crushed under high vacuum to release and (Q1 and Q2a; Table 2); sample AZ01-BM-126 was collected 6 m measure mantle helium contained in inclusions. Other mineral sepa- below the surface elevation of Surface A (T1). These sample sites were rates were hand-crushed to a fine powder and melted under high vacu- in natural near-vertical exposures on cliff faces. Samples from these um at 1400°C in a double-walled modified Turner furnace. The released sites received cosmic-ray exposure, albeit partially shielded from local gas was purified using getters and cryogenic traps. Isotopic measure- topography, only since the cliff face formed. We hypothesized that ments were made on a mass spectrometer fitted with an electron mul- 3 these samples would provide an estimate of inherited exposure tiplier and pulse counting electronics. The production rate of Hec in retained in subsurface clasts. The calcrete cement in these fans made olivine has been well-established and is the highest rate of any cosmo- it impossible to dig pits to obtain buried samples completely shielded genic nuclide (Goehring et al., 2010 and references therein; Fenton and Table 2 3Hec exposure ages of boulders collected from alluvial-fan terraces of the Black Mountains.

a a,b 3 6 e 3 Sample Latitude (N) Longitude (W) Elevation (m) B-axis diameter Height ofboulder Mass (g) R/Ra fusion R/Ra Measured total Hec (10 at/g) J ( Hec)r of boulder (cm) surface above corrected 4He±errorc z=0 cmd (at/g/yr) age±error surrounding fan (1012 at/g) (ka)f terrace (cm)

Surface A: T1 AZ01–BM–120 34.8224 114.3627 433 40 20 0.1276 131.93 131.92 1.95±0.10 369.64±18.48 150 2500±300 AZ01–BM–121 34.8224 114.3627 433 40 20 0.2723 590.49 590.48 0.44±0.02 378.04±18.90 150 2500±300 AZ01–BM–126g 34.8233 114.3643 430 40 – 0.1081 164.40 164.38 1.12±0.06 264.31±13.22 150 –g Surface B: Q1 L00–3–BT2 (oliv) 34.8346 114.4306 360 10 10 0.1344 508.68 508.67 0.33±0.02 238.29±11.91 141 1700±200 L00–3–BT4 (pyx) 34.8346 114.4306 360 10 10 0.2360 46.50 46.49 1.48±0.07 99.08±4.95 137 730±90

L00–3–LP1* (oliv) 34.8346 114.4306 360 10 10 0.0517 129.08 129.07 1.00±0.05 185.07±9.25 141 1300 ±200 86 (2013) 79 Research Quaternary / Pelletier J.D. Fenton, C.R. L00–3–BT3 (oliv) 34.8346 114.4306 360 10 10 0.2671 80.66 80.65 2.38±0.12 275.94±13.80 141 2000±200 L00–3–2 m1 (oliv)g 34.8347 114.4304 318 10 10 0.0649 5.52 5.51 6.84±0.34 54.21±2.71 137 –g L00–3–2 m (oliv)g 34.8347 114.4304 318 10 – 0.0406 2.80 2.79 14.82±0.74 59.33±2.97 137 –g Surface B1: Q2a L00–2–BT1 (oliv) 34.8109 114.4181 340 10 10 0.7540 52.74 52.73 2.70±0.13 204.80±10.24 139 1500±200 L00–2–BT2 (pyx) 34.8109 114.4181 340 10 10 0.2130 9.15 9.14 9.35±0.47 122.92±6.15 134 920±110 L00–2–LP* (oliv) 34.8109 114.4181 340 10 10 0.1806 13.50 13.49 2.19±0.11 42.55±2.13 139 310±40 Surface J: Q2a AZ01–BM–106 34.8633 114.4378 332 40 30 0.3953 39.86 39.85 1.34±1.34 76.91±3.85 138 560±70 AZ01–BM–105 34.8633 114.4378 332 40 30 0.1678 43.19 43.18 1.26±1.26 78.20±3.91 138 570±70 L00–2–2 m (oliv)g 34.8105 114.4174 318 –– 0.0722 2.94 2.93 13.27±0.66 55.85±2.79 137 –g Surface C: Q2a 020803–01 (cpx) 34.8600 114.4374 317 40 40 0.1076 0.29 0.27 47.13±2.36 18.32±0.92 132 140±20 020803–02 (cpx) 34.8601 114.4378 318 40 20 0.1066 3.31 3.29 4.70±0.23 22.25±1.11 132 170±20 020803–03 (oliv) 34.8601 114.4375 318 40 40 0.0548 5.03 5.01 2.44±0.12 17.58±0.88 137 130±20 Surface O: Q2b AZ01–BM–101 34.8687 114.4389 337 60 50 0.2678 0.89 0.87 33.83±1.69 42.37±2.12 139 310±40 AZ01–BM–103 34.8687 114.4389 337 30 30 0.0693 7.77 7.75 1.19±0.06 13.29±0.66 139 96±12 Surface H: Q2c

AZ01–BM–136 34.8619 114.4372 335 50 40 0.3426 5.96 5.94 6.62±0.33 56.64±2.83 139 410±50 – 99 AZ01–BM–135 34.8619 114.4372 335 50 40 0.4144 0.50 0.48 38.78±1.94 26.67±1.33 139 190±20 020903–09 (cpx) 34.8617 114.4376 313 30 20 0.0998 4.82 4.80 2.40±0.12 16.60±0.83 133 130±20 Surface G: Q3 020903–04 34.8617 114.4378 307 20 20 0.1815 0.32 0.30 17.92±0.90 7.74±0.39 132 56±7 Surface F: Q4 AZ01–BM–138 34.8619 114.4372 305 40 5 0.2662 11.92 11.90 1.13±0.06 19.32±0.97 138 140±20

a −6 Ra =1.384×10 . b3 4 He/ He ratios were standardized against the SIO-MM standard at 16.45 RA. All values were corrected for interference peaks, instrumental and extraction blanks (Poreda and Cerling, 1992). Corrected R/Ra results from subtracting 0.0113 from R/Ra fusion. This value is the average predicted R/Ra (where P3/P4 =2.77e-08) resulting from radiogenic and nucleogenic helium present in olivines and pyroxenes from basalts in the Black Mountains. See Supplementary Tables 2 and 3 for details. Values are based on equations of Andrews (1985) and Li, U, Th, and major element concentrations of the minerals extracted from the basalt boulder samples. c Analytical error≤5%. d3 Hec atom concentration corrected for self-shielding. e Production rates are from Fenton et al. (2009), which are corrected for mineral composition and are based on the Cerling and Craig (1994) production rate (115 atom/g/yr at sea level and high latitude). Production rates of cosmogenic 3He is 118 and 114 at/g/yr in olivines and pyroxenes, using Lal's (1991) scaling factors in CosmoCalc (Vermeesch, 2007); rates are corrected for latitude, elevation, and skyline shielding (resulting from surrounding topography). f 1σ uncertainty accounts for 5% uncertainty in production rate and 5% analytical error (corrections for precision of mass spectrometer, blanks and standards), and 10% uncertainty for P3/P4 corrections. g Sample collected at 2 m below surface in a vertical exposure in terrace. – indicates no age was calculated, because the cosmogenic nuclide concentration indicates previous exposure and/or incomplete shielding of subsurface sample. 93 94 C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99

3 Niedermann, 2012). The sea-level, high-latitude Hec production rates possible. In general, however, the inherited component in these of 118 and 114 atoms/g/yr are used for olivine and pyroxene, respec- younger surfaces, if the ca. 140 ka is an accurate estimate, could be 3 tively (Table 2; Fenton et al., 2009). Results from analyses are listed in between 25% and 87% of the measured Hec in surface boulder sam- Table 3. ples. We are aware there is likely an inherited component in each boulder sample, and that it may be greater or less than ~140 ka. 3 Major and trace-element measurements Nonetheless, we have chosen to report the concentrations of Hec and their related exposure ages listed in Table 2 as raw data. Given Whole-rock basalt and mineral-separate samples were sent to Act that we later report age ranges for the surfaces for which we have Labs, Inc. (Tucson, AZ) for determination of major and trace-element data, we conclude that the previous exposure history is accounted concentrations. Concentrations of U, Th, Li, Si, Al, Mg, Ca, and Na were for in these broad ranges (Table 3; Figs. 6 and 8). of most interest in order to estimate the amount of 4He produced in the minerals through U–Th decay (see Supplementary Tables 2 and 3; Exposure ages of fan terraces Andrews, 1985). Our raw data (Table 2) indicate a general decrease in exposure ages Results from older, higher fans to younger, lower fans within the Warm Springs fan complex representing a range from 56 ka to 2.5 Ma (Figs. 6 and 8). Inherited cosmogenic 3He Erosion of a fan surface can cause exhumation of shielded/buried clasts, which allows that ages from the older fan Surfaces A, B, and B1 are min- Samples collected from Surfaces A, B, and J (T1, Q1, and Q2a) con- imum values, due to the amount of erosion these surfaces have experi- 3 tain 254, 52, 57, and 54 Mat/g of Hec. Production, and therefore con- enced. Previous exposure and reworking of older clasts, with longer centration, of cosmogenic nuclides decreases exponentially with exposure histories, into younger fan deposits often complicates appar- depth (Gosse and Phillips, 2001). In a stable surface, the production ent exposure-age interpretation, as illustrated in data from Surfaces B, rate at 2 m depth is approximately 2% of the production rate at the B1, O, and H (Q1, Q2a, Q2b, and Q2c; Fig. 8). Compounding this compli- 3 4 3 surface (Cerling and Craig, 1994). The Hec contents in cliff-face sam- cation is the presence of radiogenic He and nucleogenic He in Black ples from the T1, Q1, and Q2a fans are 70%, 22–55%, and 70%, respec- Mountain basalts (15–20 Ma), which are the result of the relatively tively of the cosmogenic nuclide inventory in clasts collected from the high Li contents and the decay of U, Th, and their daughter isotopes. surfaces of these fans. Based on these higher-than expected ‘subsur- Fortunately, we can calculate the amounts of radiogenic 4He and 3 3 face’ Hec concentrations, we conclude that samples collected from nucleogenic He in the total helium inventory based on equations de- cliff faces were not shielded enough from cosmic-ray exposure to rived by Andrews (1985) and the estimates of Li, U, and Th contents 3 make them useful in calculating an average inherited Hec compo- of basalt samples collected in the alluvial fan complex (Supplementary nent. At most, we can conclude that there is a maximum of ca. Tables 2 and 3). These calculations lead to a more accurate determina- 3 3 1.7 Ma inheritance for clasts in the T1 surface, and ca. 400 ka maxi- tion of cosmogenic He amounts, and thus Hec exposure ages. The mum inheritance in the Q1 and Q2a surfaces. following discussion treats the results in order from oldest to youngest. Sample AZ01-BM-138 from the active Five-Mile Wash (Surface F) 3 6 contains approximately 19 Mat/g of Hec (Mat=10 atoms). This Surface A (T1) represents ca. 140 ka of previous exposure. This inherited component Two basalt boulders from Surface A, the oldest, highest fan terrace 3 is between 5 and 19% of the total Hec inventory in samples from Sur- (T1) that we sampled (Fig. 3), each yielded exposure ages of 2500± faces A, B, and B1 (T1, Q1, and Q2a) in this study, except for sample 300 ka, and this minimum age agrees with other age constraints in 3 L00-2-LP, which could have up to 45% inherited Hec. In surfaces the area. If the inherited component is accepted to be ca. 140 ka, this younger than Q2a, there are instances where the calculated inherited is within the uncertainty of the measurements. This (2500±300 ka) 3 component exceeds the total Hec inventory, which is not physically is considered a minimum age, because there is abundant evidence of erosion. There is no “unconsolidated material” at the surface of this ter- Older Younger race; instead there is a well-developed (stage IV+) Bk horizon at the surface, which has stabilized the surface with a thick calcrete cement. Q2a Q2b Q2c Q3 Q4 The ages of these clasts likely represent the time since erosion exhumed the calcrete horizon on which boulders are sitting. 20 The T1 fan (Surface A) caps Bullhead Alluvium that was deposited between 4.2 and 3.6 Ma, and incision into this alluvium began by 3.3 Ma, according to House et al. (2008). These ages are based on the 4.1-Ma age of the Lower Nomlaki Tuff, which is present in correlative, intercalated Bullhead Alluvium and Black Mountain fanglomerates at horizon (cm)

v an elevation of 230 m above present-day river level and the 3.3-Ma Nomlaki Tuff in younger Black Mountain fanglomerates near Laughlin, 10 Nevada (House et al., 2008; Fig. 2). Incision into the alluvium would eventually cause abandonment of the T1 surface. Based on the eroded surface (Supplementary Table 1), stratigraphic relations, and the minimum exposures ages, we estimate that the age of the T1 fan (Surface

Thickness of A A) is between 2.2 and 3.3 Ma (including uncertainty).

Surfaces B (Q1) and B1 (Q2a) 3 0 Hec ages of individual basalt boulders collected from Surface B C J O DIK N M E H GLF range from 730±90 ka to 2000±200 ka (Table 2; Fig. 8). These ap- Surface from this Study parent exposure ages corroborate the younger geomorphic position of Surface B (Q1) relative to Surface A (T1; Fig. 3), but it is difﬁcult Figure 7. Measured average thickness and uncertainty (n=5 for each surface) of the to precisely constrain the age of the fan surface, which like T1 has un-

Av horizon in Q2a, Q2b, Q2c, Q3, and Q4 alluvial-fan terraces in the Warm Springs Complex. dergone erosion of “unconsolidated material” down to its calcrete C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99 95

Table 3 Ages of alluvial and river gravels in the upper and lower Colorado River basin.

Fan-terrace surfaces Bull's (1991) Alluvial fans in Mohave Alluvial fans Black Mtns. AZ Mohave Possibly correlative with Colorado River from this study naming convention Desert Bull (1991) (ka) Desert from this study (ka)a terrace of House et al. (2005, 2008)

G Early Holocene (Q3) 8–12b b63 (56±7) Emerald (Lower Colorado River deposits) H Late Pleistocene (Q2c) 12–70b b150 (130±20 190±20 410±50) O Middle Pleistocene (Q2b) 70–200b, c ca. b350 (130±20 140±20 170±20 Chemehuevi Beds (ages range from 32–102 ka; 96±12 310±40) must be b780 ka based on normal polarity C J B1 Middle Pleistocene (Q2a) 400–730b, c ca. 110–1100 (130±20 140±20 Riverside beds (1.5–1.7 Ma)d 170±20 560±70 570±70 310±40 920±110 1500±200) B Early Pleistocene (Q1) 1200b bageb3000e ca. 1100–2200 Incision to near modern river level between 3.3 (1300±200, 1700±200, 2000±200 730±90) and 1.7 Ma A Tertiary (T1) ca. 2200–3300 (2500±300; b3300; Bullhead alluvium (aggradation: 3.6–4.2; incision House et al., 2008) begins 3.3 Ma)

Note: Ages in bold are age ranges determined in this study based on our limited cosmogenic exposure ages and the relative stratigraphic positions of Surfaces A through O. a 3 Cosmogenic Hec ages. b Based on development of soil carbonate. c Based on U−series ages. d House et al. (2005) report “minor aggradation ca. 1.5–1.7 Ma (Riverside Beds), adding that the Riverside Beds are the first Lower Colorado River deposits laid down following incision of the Bullhead Alluvium and the Laughlin Conglomerate. e Age of basalt flow underlying gravels. layer (Supplementary Table 1). Previous exposure (ca. 140 ka) would fan near Phoenix, AZ that experienced aggradation from 1000– have little effect on the exposure ages. There is overlap (within 2σ)of 1500 Ma (unrelated to the ~2 Ma also mentioned in their study). This the three oldest ages (1300, 1700, and 2000 ka), and the youngest fan may record the similar events preserved in the B1 fan of the present age (730 ka) might represent an exhumation age. The Q1 fan does study. House et al. (2005) reported aggradation of the Riverside Beds as have Stage IV carbonate development with continuous laminations the first river deposits laid down in the Lower Colorado River corridor up to 8 cm thick (Table 1), which indicates considerable antiquity following incision of the Bullhead Alluvium and the Laughlin Conglom- (after Machette, 1985). erate. House et al. (2008) state, in point 8 of their working model of the House et al. (2005) mapped Surface B as their C2 fan, which “uncon- Tertiary and Quaternary history of the Lower Colorado River, that minor formably overlies dipping beds indicating older Pliocene(?) deforma- aggradation occurred ca. 1.5–1.7 Ma and they label this aggradation as tion.” Surface B (Q1) is 86 m below and inset against the T1 (Surface the Riverside beds. It is possible that our B1 surface represents deposi- A) remnant. It is possible clasts worked on the Q1 surface may have ex- tion post-dating the Riverside Beds (Table 3). perienced exposure to cosmic rays while previously on the T1 fan. It is also possible, however, that the oldest clast (2000±200 ka) on the Surface J (Q2a) Q1 fan represents an age estimate of abandonment of the Q1 surface. Surface J, another Q2a fan remnant, which is stratigraphically and Robinson et al. (2000) reported a Pleistocene fan near Phoenix, AZ geomorphically younger than Surface B (Q1), is preserved on the that experienced aggradation around 2 Ma. Elsewhere, House et al. north side of Five-Mile Wash (Figs. 4 and 6). This remnant yielded 3 (2005) suggested that the Bullhead Alluvium, and thus the T1 surface, two cosmogenic samples with Hec ages of 560±70 and 570± was deeply incised between 3.3 and 1.7 Ma, and that a “series of fans 70 ka (Table 2). The ages are indistinguishable and even considering and river terraces” that were deposited during that time preserve the possibility of previous exposure history, it is unlikely that the Stage V+ and IV carbonate horizons. The Q1 fan terrace of the present two clasts would have exactly the same history prior to deposition. study could possibly be coeval (Table 3). The bottoms of clasts on Surface J have carbonate coatings in pendant The Surface B1 fan remnant (Q2a) is adjacent to and stratigraphically form that are 1–3 cm thick, and there is Stage V soil carbonate pres- younger than the Surface B, found to the north (Fig. 3). The Surface B1 ent on Surface J. The Surface J fan terrace shares a drainage with the terrace shares a drainage with the T1 fan. Surface B1 is mapped by Q1 and T0 fan terraces (Fig. 3), from which pendant sediments may House et al. (2005; in their Fig. 20) as a C3 "old fan," which post-dates have been transported. There was no naturally-occurring vertical ex- the Bullhead Alluvium and is younger than their C2 fan (Surface B in posure of the soil profile to further investigate soil-carbonate devel- this study). Pearthree et al. (2009) mapped a fault scarp, labeled as opment. Surface J is likely younger than 1.1 Ma, based on its ‘scarp’ in Figure 3, that cuts across the Surface B1 (Q2a) fan deposit, not- stratigraphic position and on the youngest exposure-age estimate of ing only that it represented a structural perturbation of “early to middle Surface B (Q1). Pleistocene” fan deposits. 3 Hec ages of clasts collected from the Surface B1 are 310±40 ka, Surfaces C (Q2a) and D (Q2b) 920±110 ka, and 1500±200 ka (Table 2; Fig. 8). This is a considerable Three basalt boulders were collected from fan Surface C (Q2a), range in exposure ages, and there was no naturally-occurring vertical ex- which is inset against and adjacent to the stratigraphically older Q1 posure of the soil profile to reveal soil carbonate development. Rare, thick (Surface B; Figs. 3 and 4). These boulders yielded an overlapping cluster soil-carbonate pendants are found scattered on the surface (Table 1). of exposure ages at 130±20, 140±20 and 170±20 ka (Table 2). It is Stratigraphically Surface B is younger than Surface A (2.2–3.3 Ma), clear that these ages cannot include 140 ka of previous exposure, as and older than Surface B1. Based on this physical relation, we conclude this surface is mapped as Q2a and has geomorphic characteristics to that sample L00-3-BT4 (730±90 ka) is likely an outlier, representing match. It is possible that the three exposure ages represent exhumation an exhumation age, and that the exposure age estimate for Surface B ages, and are thus only considered to be minimum ages. There is no ver- (Q1) is ca. 1.1–2.2 Ma. Likewise, exposure-age estimates for the Surface tical exposure of the Surface C fan terrace; however, the adjacent and B1 (Q2a) fan are 810 ka to 1.1 Ma (including uncertainty), assuming geomorphically younger Q2b fan terrace (Surface D) preserves a small sample L00-2-LP (310±40 ka) represents an exhumation age and is cliff-face. The vertical exposure of undated Surface D (Q2b), only 1 m considered an outlier. Robinson et al. (2000) reported a Pleistocene below Surface C, shows that the clasts are completely coated in soil 96 C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99

Interglacial A

Glacial 63 Marine Oxygen Isotope Stages (Early Pleistocene) Q2c Q2b Q2a Q1 B (exhumed?) 1100 ka < age < 2200ka Surface B (Q1) (exhumed?) 810 ka < age < 1100ka Surface B1 (Q2a) 490 ka < age <1100ka Surface J (Q2a) 110 ka < age <1100 ka Surface C (Q2a) <350 ka

g Stratigraphic Age Surface O (Q2b) <150 ka (inheritance?) Surface H (Q2c) Q2c AlluvialFan Units with Time Intervals (inheritance?) According to Bull’s (1991) scheme 3 Increasin He exposure ages

Alluvial-fan Terraces (this study) Alluvial-fan Terraces Surface G (Q3) <63 ka c Age outlier

(Holocene) 0 200 400600 800 10001200 1400 1600 1800 Time (ka)

Figure 8. Data showing (A) a composite oxygen isotope curve for the past 1.8 Ma, constructed by cross-correlating 57 globally distributed benthic marine δ18O(‰) records (mod- iﬁed from Walker and Lowe, 2007). Glacial and interglacial marine-oxygen isotopic stages are denoted by even and odd numbers, respectively. (B) Cosmogenic exposure ages of alluvial-fan surfaces from this study are plotted against their relative stratigraphic age. Bull's (1991) mapping units and time intervals assigned are given by boxes with dashed lines. Comparison of exposure age data from this study and marine oxygen isotope stages for the past 1.8 Ma illustrates that exposure age dating of alluvial fans is not yet accurate enough to assign a speciﬁc marine oxygen isotope stage to the time during which deposition and/or incision of alluvial fans in the Warm Springs Complex occurred.

carbonate and the matrix and clasts are cemented together, indicating Surface G (Q3) Stage II-III soil-carbonate development. Rare, discontinuous, thin car- One boulder sample taken from the surface of the Q3 fan remnant bonate laminations occur approximately 1.5 m below the fan surface. (Surface G) yielded an exposure age of 56±6 ka (Table 2). This rem- The Q2b (Surface D) fan surface is stratigraphically younger than Sur- nant is 1.5 m above the active wash and is stratigraphically and face C (Table 1), and thus likely younger than ca. 110–190 ka, the geomorphically younger than Surface H (Q2c; Figs. 4 and 6). The ex- range of exposure ages on the Q2a (Surface C) fan. Surface D did not posure ages of both surfaces, at least nominally, are consistent with yield any boulder samples suitable for cosmogenic 3He dating. this. It is difﬁcult to assess the accuracy of the age of Surface G, as there were no soil/soil carbonate exposures on the Q3 terrace. It is Surface O (Q2b) possible there is some previous exposure recorded in the clast. At Two basalt boulders were collected and analyzed from Surface O, best, we can say that this terrace must be less than 63 ka (including which is preserved north of the Five-Mile Wash drainage (Figs. 4 uncertainty). and 6). These boulders yielded exposure ages of 310±40 and 96± 10 ka (Table 2). Clast bottoms at the surface have very thin, weak car- Discussion and conclusions bonate coatings. Stratigraphically, Surface O has been mapped as a possible Q2b surface. The scatter in the ages and the poor develop- Basalt clasts on alluvial fans in the Black Mountains yield cosmogen- 3 ment of soil carbonate suggest that the fan is young, and the oldest ic Hec ageestimatesthatprovidequantitativeestimatesofQuaternary 3 exposure age is likely affected by previous exposure. At best, we con- alluvial fan and terrace ages. The Hec age estimates generally conﬁrm clude the fan surface is b350 ka (including uncertainty). alluvial fan terraces become younger with decreasing relative stratigraphic age (Figs. 6 and 8; Table 3). Fan-terrace Surfaces A and B (T1 Surface H (Q2c) and Q1) have exposure age estimates that decrease with decreasing The Q2c fan (Surface H) is stratigraphically younger than Surfaces D soil carbonate development and other relative geomorphic age indica- and O, Q2b fan remnants (Table 1; Figs. 4 and 6). Three boulders collect- tors, AZ (Tables 1 and 2; Supplementary Table 1). Exposure ages of ed from Surface H just to the north of Five-Mile Wash yielded exposure older surfaces are more affected by erosion and restabilization of the ages of 130±20, 190±20, and 410±50 ka (Table 2; Fig. 8). The youn- surface, thus yielding ‘minimum’ old ages, rather than by previous expo- gest of these three ages overlaps the ages of clasts on Surface D (Q2b); sure, which varies greatly and is on the order of ~140 ka based on this however, clast bottoms in the Q2c fan (Surface H) have only Stage II study and data reported in Gosse (2003), Nichols et al. (2002),and coatings, which are often discontinuous. The terrace is only 3.5 m Clapp et al. (2000, 2002). Cosmogenic analysis of a basalt clast from above the active wash and the carbonate development indicates that the active wash shows that previous exposure (perhaps ~140 ka) is in- the fan is young. All three clasts may record previous exposure histories, deed a concern for alluvial fans, but it has a dominant effect on younger 3 or possibly, the youngest age (130±20 ka) on Surface H might repre- (bQ2a) surfaces, where the inherited Hec component is a larger per- 3 sent the maximum possible exposure age of this surface. centage of the total Hec inventory measured in a clast. C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99 97

Three surfaces mapped as Q2a (Surfaces B1, J, and C) have a consid- western USA can be correlated regionally (Christensen and Purcell, erable range of exposure ages (Table 2) representing a mixture of previ- 1985; Wells et al., 1990; Bull, 1991; Morrison, 1991; Gillespie et al., ous exposure history, as well as erosional and exhumational processes. 1994; Swadley et al., 1995; Bull, 1996; Reheis et al., 1996). Several If all three surfaces are in fact Q2a in stratigraphic age and are consid- of these studies (Whitney et al., 2004 and references therein) have ered to be surfaces that were contemporaneously deposited, we can documented fill events and/or development of surface exposures 3 only conclude that based on our Hec exposure age estimates the Q2a between 2000–900 ka, 700–320 ka, 150–120 ka, 90–50 ka, 7–20 ka, deposits were emplaced sometime between 110 ka and 1.1 Ma. and 0–8 ka. Regional similarity of ages has provides strong support Based on stratigraphic relations and exposure age estimates, Q2b, for climatic control of Quaternary alluviation. Q2c, and Q3 surfaces in the Five-Mile Wash are perhaps b350 ka, Particularly accurate age control provided by Weldon (1986) for b150 ka, and b63 ka, respectively. Samples collected from Surface H terraces in Cajon Creek, CA, and Reheis et al. (1996) in Fish Lake Val- (Q2c) have a large scatter in ages, but the surface is geomorphically ley, Nevada suggest that pulses of aggradation in the southwestern younger than the T1, Q1, and Q2a terraces. It is likely that clasts in USA accompany humid-arid climatic transitions recognized in paleo- the younger Surfaces O, H, and G (Q2b, Q2c, and Q3) are affected by climatic proxies such as the Owens Lake (Smith et al., 1997; Mensing, previous exposure to cosmic rays, but it is impossible to quantify ex- 2001) and San Felipe Basin (Guerrero et al., 1999) sediment cores. actly how much in this study. Inheritance, or reworking of older clasts Bull (1991, 1996) suggested that humid–arid transitions resulted in into younger terraces, is the dominating control on ages of younger a vegetative succession from mature woodland vegetation to desert fan surfaces, whereas erosion is the dominating control on older fan shrub over large areas in the southwestern USA, which increased hill- surfaces. The influence of previous exposure histories diminishes slope sediment supply during humid–arid transitions; cooler, wetter with increasing depositional age of a fan (Gosse, 2003); clasts in periods allowed for increased bedrock weathering and generation of 3 older terraces (>600 ka) likely have a component of inherited Hec hillslope sediment that was “held in place” by greater vegetation but it has a much smaller effect on the apparent exposure age. cover. This hypothesis suggests that variations in hillslope sediment The large uncertainties in exposure ages preclude temporally supply resulting from changes in vegetation cover are more impor- linking Black Mountain alluvial fans to specific humid-to-arid transi- tant than changes in the mean or variance of hillslope runoff during tions (Fig. 8). recognized in other regions of the Southwest USA. The these climatic changes, and there is significant variability in hillslope morphology of the alluvial fans (Fig. 1B) and lack of significant responses in the southwest USA (Harvey, et al., 1999; Nichols, et al., Plio-Quatenary faulting/folding in the area (Pearthree et al., 2009), 2002; McDonald, et al., 2003). Anders et al. (2005) demonstrated in however, suggest that episodic increases in sediment supply possibly the upper reaches of the Colorado River, that the mainstem Colorado linked to climate change have played a role in triggering Late Cenozo- River and the local tributary catchments of eastern Grand Canyon ap- ic piedmont and valley-floor aggradation in the LCR valley. Pulses of pear to have differently timed responses to climate change, at least Late Quaternary deposition in the southwestern USA terminating at over the past 400 ka. They further conclude that local tributary drain- ca. 320, 120, 50, and 15–7 ka correlate with humid-to-arid transitions ages are linked to hillslope sediment sources, but record significant recognized in paleoclimatic proxies (e.g. Ku et al., 1979; Weldon, lag times in responses to climate change. Weathering-limited hill- 1986; Reheis et al., 1996; Zehfuss et al., 2001; McDonald et al., slope sediment supply may be the limiting factor in eastern Grand 2003; Anders et al., 2005). The data and proposed interpretations Canyon. A more complete understanding of desert landscape response presented in this paper provide further support to this hypothesis. to climate change is still needed. Cosmogenic ages of alluvial fans near the Black Mountains are only We tentatively correlate our Black Mountain fan-terrace surfaces age estimates. An independent dating technique, such as 230Th/U ages to those of Bull (1991) and assign age estimates as follows (Table 3): of b600 ka soil carbonate rinds, for example, would be a welcome and T1 = Surface A (3.3–2.2 Ma); Q1 = Surface B (2.2–1.1 Ma); Q2a = very suitable technique for cross-checking the exposure ages and to Surfaces B1, J, and O (1.1 Ma >age >110 ka); Q2b = Surface C clarify the complex chronology of alluvial-fan aggradation in the (b350 ka), Q2c = Surface H (b150 ka); and Q3 = Surface G (b63 ka). Lower Colorado River corridor in this region. The broad range of pres- Relative stratigraphic ages of these fan terraces, particularly of the ently available ages within the younger fan surfaces described herein, younger surfaces, are more reliable than the cosmogenic age estimates. as well as the probability of reworked clasts with prior exposure histories, do not yet allow a rigorous test of the hypothesis that Pleistocene Acknowledgments hillslopes and tributaries to the Lower Colorado River responded syn- chronously with the main-stem river during major climate changes. We gratefully acknowledge field support and/or critical discus- Our data from the T1 fan terrace (Surface A) does support the sions with Leslie Hsu, Keith Howard, Kyle House, Phil Pearthree, and hypothesis of House et al. (2005, 2008) that incision into the Bullhead Brenda Buck. We thank Thure Cerling for use of the mass spectrome- Alluvium and overlying fan gravel in the Lower Colorado River corridor ter at the University of Utah. Support was granted by the Gladys began after 3.3 Ma. The deposition and abandonment of this old fan sur- W. Cole Memorial Research Award received by C. Fenton in 2002, face is probably linked more to the inception of the Lower Colorado the National Research Council (through a Research Associateship for River as proposed by House et al. (2005, 2008) and less to transitions Fenton 2002–2004), and the National Science Foundation award in climate change. Deposition of the next younger Surface B1 (Q2a) #0309518 to J.D.P. We wish to thank associate editor Jim Knox and may overlap in time with a period of Colorado River aggradation of the two anonymous reviewers for their comments, which greatly im- Riverside Beds between 1.5 and 1.7 Ma, as proposed by House et al. proved the paper. (2005). 3 Though comparison of Hec age data from this study and marine Appendix A. Supplementary data oxygen isotope stages over the past 1.8 Ma (Fig. 8; Walker and Lowe, 2007) illustrates that exposure-age dating of clasts found on Supplementary data to this article can be found online at http:// the surfaces of alluvial fans is not yet accurate enough to assign spe- dx.doi.org/10.1016/j.yqres.2012.10.006. cific glacial or interglacial periods to the time during which deposition or incision of alluvial fans in the Warm Springs Complex References occurred, we still infer that fan surfaces younger than Q1 are more likely related to shifts in regional climate and hillslope-sediment sup- Anders, M.D., Pederson, J.L., Rittenour, T.M., Sharp, W.D., Gosse, J.C., Karlstrom, K.E., Crossey, L.J., Goble, R.J., Stockli, L., Yang, G., 2005. Pleistocene geomorphology and ply. A growing database of surface and stratigraphic ages suggests geochronology of eastern Grand Canyon: linkages of landscape components during that Quaternary geomorphic surfaces and underlying deposits of the climate changes. Quaternary Science Reviews 24, 2428–2448. 98 C.R. Fenton, J.D. Pelletier / Quaternary Research 79 (2013) 86–99

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